1. Field of the Invention
The present invention relates to semiconductor packages, and more particularly to packages in which one or more dies are mounted on a flange within an opening in a window frame mounted on the flange and having leads mounted thereon.
2. History of the Prior Art
It is known in the art to provide semiconductor packages in which one or more semiconductor dies are mounted on a heat sink flange within an opening in a window frame which mounts and insulates a plurality of leads. The dies may be of the LDMOS (lateral diffusion metal oxide semiconductor) type and the package of the type for packaging LDMOS power transistors. The window frame, which is typically made of ceramic based materials such as alumina, serves to mount the leads on the semiconductor package and insulate the leads from the heat sink flange and other portions of the package. The window frame has an opening therein which surrounds the semiconductor dies. The dies are electrically coupled to the conductors such as by wire bonds.
In semiconductor packages of the type described, the components parts thereof, including the flange, the window frame and the leads, are joined together such as by brazing/soldering to form a header. One or more dies are then mounted such as by brazing/soldering/adhering to the header and are electrically attached to the leads such as by wire bonding. During assembly of the header, the semiconductor package is typically subjected to very high temperatures on the order of 700-900° C. in order to carry out brazing. Such high temperatures dictate that the materials being joined, including particularly the heat sink flange and the window frame, have similar coefficients of thermal expansion (CTE). Ideally, the flange is made of highly conductive materials such as copper. However, because the alumina or other ceramic materials of the window frame has a much lower rate of thermal expansion, it is often necessary to make the flange of less conductive material having a closer CTE match to the ceramic material of the window frame. Without such a close CTE match, the flange and the window frame expand and contract at substantially different rates so as to place substantial stresses on the package. Complicating such situations is the brittle nature of the ceramic material of the window frame, causing it to break or otherwise fail as a result of such stresses.
It would therefore be desirable to provide a semiconductor package allowing for the use of highly conductive materials such as pure copper for the heat sink flange. The material of the window frame should provide a close CTE match with the flange, and should ideally be flexible and not brittle in order to better withstand the stresses that may occur during assembly of the package.
Further problems may arise during assembly of the header, particularly when brazing is used to join the parts together. The top surface of the flange within the opening in the window frame forms a die attach area for mounting one or more dies. Such die attach area must be smooth and free of brazing material in order to properly attach the dies thereto. However, during assembly of the header, the brazing material at the interface between the window frame and the flange may flow into the die attach area so as to interfere with the subsequent mounting of the die within such area. It would therefore be desirable to prevent such brazing material from flowing into the die attach area during assembly of the header.